The support of buildings on piles is of course a well established practice and the drilling and installation of piles is a highly competitive business required careful attention to costings. Yet further recent more attention to environmental issues has led to increased problems in driving piles into the ground particularly in respect to vibration damage and in respect of pollution generated by relatively crude impact hammers.
As is well established, very heavy structures are supported on piles which are drilled to bedrock. In many cases drilling to bedrock is an expensive requirement since the depth can be relatively long.
In intermediate structures providing a loading on each pile generally in the range 350 KN to 2400 KN (78,000 to 539,000 lbs.) it is generally not a requirement that the pile extend to bedrock and attempts are made to ensure that the pile is supported on sufficient level of compressed material to allow the required loading without the pile moving downwards under the loading. Well known testing procedures are available for testing the loading of a particular pile structure.
Many areas involve a soil construction which has an overlay of sedimental clay sitting on top of a layer of glacial till. Conventionally piles for mounting in this structure of soil require that the pile extend through the overlay of clay into the underlying till and extend into the till a sufficient distance for compression of the till to occur in the area underlying the bottom of the pile. The installation of such piles is generally carried out in the above soil conditions by driving a pre-cast concrete pile downwardly through the clay and into the till to a sufficient distance that the till is compressed. Driving is carried out generally by a hammering action on the top of the pile either using a simple crude drop hammer which is raised and lowered on a winch or by more technically advanced mechanical hammers which are held stationary above the pile and which apply a hammering action to the top of the pile.
These systems of applying impact forces to the pile have a number of significant disadvantages. Firstly the manufacture of the precast concrete pile is very expensive and problematic so that the installer requires to rely upon the manufacturing schedule and costings of a pile supplier which may be unsatisfactory. Secondly the action of impacting upon the pile leads to significant shockwaves traveling through the ground at the upper levels of the ground. These shockwaves are potentially damaging to adjacent structures so that such structures need to be protected. Even where no damage actually occurs, there is a risk that owners of adjacent structures have the perception that their structure has been or may be or is being damaged and so may raise complaint.
Another form of pile installation shown for example in a brochure by IHC Fundex Equipment of The Netherlands is known as a displacement pile or “soil displacement drilling” in which a tubular structure with a shoe at the lower end is inserted into an initial drilled hole where the bottom of the drilled hole terminates at the bottom of the clay layer. With the tubular member inserted into the hole, the tube member with the attached shoe is driven further into the ground by simultaneous downward compression forces and reciprocating rotational movement or rotational movement in a single direction. The shoe is shaped and arranged to act to break up and disperse the material in the till layer underneath the bottom of the tubular structure so that the vertically downward forces on the tubular structure can drive the tube and the shoe downwardly into the till.
However this technique is not widely accepted since the depth of penetration is very much limited before the compaction of the till underneath the shoe prevents further downward movement. The compaction achieved in this way is thus only sufficient for loading of the order of 270 KN which is generally unsatisfactory for conventional building techniques and structures which require higher loadings. This technique has therefore not been widely adopted except in certain specific areas, such as in Holland and Florida where land reclamation has been used providing ground conditions which are particularly conducive to this technique.
In most cases therefore where the ground conditions include the underlying glacial till and the overlying clay, the present technique is to use the above precast concrete pile which is driven by impact forces to the required depth. The impact forces provide sufficient compression of the underlying material to provide an increased level of loading up to a required amount generally necessary for buildings of this type.
However there remains a requirement for replacing the driven pre-cast concrete pile both for reasons of cost and for environmental damage.